1. Field of the Invention
The present invention relates to a process for producing a polycrystalline semiconductor film, more particularly, to a process for forming at a low temperature by ion beam induced crystal growth a polycrystalline semiconductor film having a controlled uniform grain size.
2. Description of the Prior Art
Much has been reported about the ion beam induced crystal growth. An example is the solid phase epitaxial growth of amorphous silicon on a silicon substrate by ion implantation. (J. Nakata and K. Kajiyama, Jpn. J. Appl. Phys. 21 (1982) Suppl. 21-1, p. 211, and many others.)
There has been reported some example about ion implantation into a semiconductor film on a glass substrate. This process is designed to increase the grain size of a silicon or germanium film formed on a silica glass substrate by performing silicon or germanium ion implantation while heating them. (H. A. Atwater et al., J. Appl. Phys. 64 (1988) p. 2337)
There has also been reported some example about the production of a polycrystalline semiconductor film having a controlled uniform grain size. The production process involves the formation of an amorphous semiconductor film containing a crystal phase and the subsequent selection and/or growth of the crystal phase by ion implantation.
The above-mentioned processes which employ ion beams are advantageous over other processes in that they permit the solid phase growth at low temperatures. Therefore, they are expected to make it possible to process semiconductors at a lower temperature than before. They are also expected to be applied to the production of three-dimensional integrated circuits. Particularly, the combination of ion beam induced crystal growth and plasma CVD permits the formation of a polycrystalline semiconductor film at low temperatures.
One of the above-mentioned conventional processes (which involves the formation by plasma CVD of an amorphous semiconductor film containing a crystal phase and the subsequent selection and/or growth of the crystal phase by ion implantation) has a disadvantage that the film formed by plasma CVD contains about 10% hydrogen and/or fluorine (originating from the raw material gas SiH.sub.4 or SiF.sub.4 in the case of silicon film). It has been found that hydrogen and fluorine inhibit the ion beam induced crystal growth. Thus the presence of hydrogen and fluorine poses a serious problem with crystal growth. That is, the semiconductor film containing hydrogen and fluorine becomes amorphous without crystal growth when irradiated with an ion beam under the condition that the crystal phase in a semiconductor film will grow in the absence of hydrogen and fluorine. (The condition for crystal growth is attained by heating the substrate at 50.degree.-800.degree. C. using a heater or by ion implantation which heats the silicon film to 220.degree. C. or above.)